One of the most common applications of microwave technology today is radio link communications systems. Microwave (or radiofrequency (RF)) communications systems can be used to provide communications links to carry voice, data, or other signals over distances ranging from only a few meters to deep space. At a top level, microwave communications systems can be grouped into one of two types; guided systems or radio links. For guided systems, signals are transmitted over a low loss cable or waveguide. For radio links, radio signals are propagated through space.
One such radio link communications system is a satellite communications system. A satellite communications system includes at least one communications satellite (COMSAT) and one or more terrestrial satellite communications terminals. A communications satellite is a man-made satellite, also referred to as an artificial satellite, which is placed into one of a variety of orbits, for example, geostationary, molniya, elliptical, and low earth, for the purposes of providing telecommunications.
Satellite communications systems can be used to provide a number of services, which at the top level can be classified as providing one of two types of communications services: point-to-point communications services, or broadcast communications services. In general, a communications satellite acts as a microwave radio relay, receiving uplink signals from one terminal and providing a downlink to another terrestrial terminal at a different location. Communications applications which use satellite systems include maritime, vehicular, and aviation applications, in addition to handheld devices and radio and television broadcasting.
Typically, terrestrial satellite communications terminals receive downlink signals from a satellite and, if the terminal is equipped to do so, transmit uplink signals to the satellite.
Important components in any radio link communications systems are antennas. An antenna is a component that converts a wave propagating on a transmission line to a wave propagating in free space (transmission), or a wave propagating in free space to a wave propagating on a transmission line (reception). A wide variety of antenna types and geometries exist, including aperture antennas, reflector antennas, phased array antennas, and combinations thereof.
Antennas are particularly important in satellite communications systems, as can be seen from the Friis power transmission formula:
            P      r        =                            P          t                ⁢                  G          t                ⁢                  G          r                ⁢                  λ          2                                      (                      4            ⁢                                                  ⁢            π            ⁢                                                  ⁢            R                    )                2              ;where Pr is power received, Pt is power transmitted, Gr is the gain of the receive antenna, Gt is the gain of the transmit antenna, R is the distance between the antennas, λ is the wavelength of the signal of interest and it is assumed that the main beams of the antennas are aligned. Further the effective gain, G, of an antenna can be expressed in terms of an effective area, Ae:
  G  =                    4        ⁢                                  ⁢        π        ⁢                                  ⁢                  A          e                            λ        2              .  The effective area, Ae, is directly related to the physical area, A, of an antenna by the antenna efficiency, ηa, or aperture efficiency: Ae=ηaA. According to the Friis power transmission formula, the power density received at a receive antenna is inversely proportional to the square of the distance the signal travels from the source. Because satellite communications links are used to communicate over great distances, the gains provided by the antennas are particularly important.
Aperture antennas are often flared sections of waveguide, typically referred to as a horn antenna, or are simply open-ended waveguide. Such antennas are commonly used at microwave frequencies and have moderate antenna gains. Antennas of this type are often used for aircraft and spacecraft applications because they can be conveniently flush mounted on the skin of the vehicle and filled with a dielectric material to provide protection to the aperture from the hazardous conditions of the environment while maintaining the aerodynamic properties of the vehicle.
Reflector antennas are typically used for applications requiring high antenna gain, such as satellite communications system. The high gains provided by reflector antennas are useful for increasing the range of a microwave system. Usually, the high gains provided by such antennas are achieved by focusing the radiation from a small antenna feed onto an electrically larger reflector. An antenna feed is a component of an antenna that couples electromagnetic energy (e.g., microwaves or RF waves) to or from a focusing component of the antenna structure, such as a reflector. In other words, for transmission, an antenna feed guides RF energy from a transmission line to a reflective and directive structure that forms the RF energy from the antenna feed into a beam or other desired radiation pattern for propagation in free space. For reception, the process is reversed. The reflecting structure focuses RF energy to the antenna feed, which collects the incoming RF energy. The RF energy is then propagated along a transmission line to the receiver. Often, the antenna feed is a dipole, feed horn or, simply, even an open-ended waveguide.
Reflector antennas typically include a feed and additional reflective and directive structures, such as a parabolic dish or parasitic elements, whose function is to form the radio waves from the feed into a beam or other desired radiation pattern. Often, reflector antennas are protected from the environment by enclosing them within a radome. A radome is, ideally, an electrically invisible structure that provides protection from the environment for the antenna. If the radome is electrically invisible to the antenna (or approximately transparent), it will not degrade antenna performance significantly. Radomes are typically made from low dielectric hydrophobic materials.
Reflector antennas, of which a parabolic dish antenna is a specific type, are relatively easy to fabricate and are typically quite rugged. However, such antennas can be quite large and unwieldy to move. Because of this, robust mechanical systems are typically needed to steer reflector antennas. The directive beam of a reflector antenna is typically directed along the bore sight axis of the parabolic dish and steered solely by mechanical means used to rotate or otherwise adjust the angular direction of the parabolic dish.
Phased array antennas include multiple stationary antenna elements, typically identical, which are fed coherently and use variable phase or time delay control, or a combination thereof, at each element to scan a directive beam to a given angle in space. Variable amplitude control can also be used to provide beam pattern shaping. Examples of typical phase array antenna elements, also called radiators, include dipoles, microstrip, or patch elements. The primary advantage of the phased array antenna over more traditional antenna types, such as aperture and reflector antennas, is that the directive beam can be repositioned electronically, i.e., scanned electronically. Electronic beam steering can be useful for quickly and accurately redirecting a beam.
Hybrid antennas, such as reflector antennas with a phased array feed, combine useful characteristics of both antenna types, such as the high gain and/or robust design of a reflector antenna, and the agile electronic steering capabilities of the phased array antenna. Although not typically used due to the design costs outweighing the increased performance, a hybrid reflector antenna with a phased array feed can be electronically scanned over a limited angular region.
Polarization is an important characteristic of all electromagnetic waves. Polarization describes the motion through which an electric field vector of an electromagnetic wave points as the electromagnetic wave travels through a point in space. The electric field vector tip can trace a line, circle, or ellipse as the electromagnetic wave passes through the imaginary point in space. In general, these traces are referred to as linear, circular, or elliptical polarization, respectively.
Polarization is important in many applications, and particularly for antennas. The polarization of the antenna is defined by the electromagnetic wave it radiates when the antenna is transmitting. The polarization characteristic of the antenna is important because, for maximum power transfer between radio links, the transmitting and receiving antennas must be of identical matching polarization states at the same time. If the transmitting and receiving antennas are orthogonally polarized, for example, the transmitting antenna is horizontally polarized while the receiving antenna is vertically polarized, then no power would be received. Conversely, if both the transmitting and receiving antennas are horizontally or vertically polarized, then maximum power is received. Because antennas facilitate the transition of electromagnetic energy propagating between free space and a transmission link, polarization is also an important characteristic of all antennas.
Many microwave systems, such as satellite communications systems, rely on waveguide transmission lines for low loss guided propagation of microwave power. Such waveguide systems or networks are typically used as part of an antenna feed. A waveguide, which is typically a rectangular or circular tube with conducting walls, is capable of handling high power microwave signals, but is bulky and expensive. Because waveguides include only a single conductor, they support transverse electric (TE) and transverse magnetic (TM) waves, which are characterized by the presence of longitudinal magnetic or electric field components, respectively. Waveguides are one of the earliest types of transmission lines developed to transport microwave signals and are still used today. Because waveguide technology is mature, there are a large selection of waveguide components, such as splitters/combiners, couplers, detectors, isolators, attenuators, phase shifters, and slotted lines commercially available for various standard waveguide bands from 1 gigahertz (GHz) to over 220 GHz. Due to the recent trend toward miniaturization and integration, many microwave circuits are currently being fabricated using planar transmission lines, such as microstrip and stripline, instead of waveguide. However, the performance required for many high power applications, such as satellite communications systems, and, in particular, antenna feed assemblies for such systems, necessitates the use of waveguides.
For the sake of design simplicity, most waveguide-based transmission line systems support only a single propagating mode, known as the “fundamental” mode. The single fundamental propagating mode is typically the first mode that propagates through the waveguide having a frequency above the cut-off frequency of the waveguide. Waveguides can be characterized as high pass filters as they enable signals above the cut-off frequency to propagate and attenuate all signals below the cut-off frequency. Due to the inverse relationship between wavelength and frequency, higher frequency waveguide components, such as Ku-band systems, have smaller dimensions than lower frequency components, such as C-band systems. In high-power systems, voltage breakdown or arcing can occur when the dielectric (typically air for waveguides) separating conducting walls breaks down. Such arcing is more likely to occur in high-powered, high frequency systems because of the relatively small dimensions and, thus, lower breakdown voltage, between conductors.
Typically, the fundamental mode or another low-order mode, couples well with a free-space radiating beam. In other words, the low-order mode is well matched, and, thus, transfers energy efficiently to free space. In such instances, the propagating mode represents the beam pattern at the feed horn, which illuminates the focusing reflector of a reflector antenna. Generally, the goal is to have a pure single mode at the feed to minimize beam distortion.